1. Field of the Invention
This invention relates generally to a unitary locking cap device with secures a rod in the head portion of a pedicle screw, and more particularly to a locking cap having concentric post and outer cylindrical rim portions which are rotationally freely coupled with one portion being threaded and the other being smooth.
2. Description of the Prior Art
The spinal column is highly complex system of bones and connective tissues which houses and protects critical elements of the nervous system and the arterial and veinous bodies in close proximity thereto. In spite of these complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist through a wide range of motion.
Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease, however, can result in spinal pathologies which either limit this range of motion, or which threaten the critical elements of the nervous system housed within the spinal column. A variety of systems have been disclosed in the art which achieve this immobilization by implanting artificial assemblies in or on the spinal column. The most widely utilized of these immobilzation assemblies, especially in the lumbar spine, are the posterior pedicle screw and rod assemblies. These assemblies consist of screws which are inserted into the pedicles and/or sacrum, and rods which are coupled to the heads of the screws. The heads of the screws have U-shaped receiving channels in either the top or sides thereof. In addition, a class of these devices includes screws which have head portions which are polyaxial inasmuch as the heads can be angulated with respect to the shaft. One such polyaxial pedicle screw is described in U.S. Ser. No. 08/559,196, entitled "A Polyaxial Locking Screw And Rod Coupling Element", filed Nov. 13, 1995. In the devices of this class, it is generally the case that the insertion of the rod in the U-shaped channel, and subsequent locking of the rod therein causes the head and the shaft to be locked in position relative to one another. The locking of the rod in the channel is accomplished by the application of a threaded nut onto the outer top of the U-shaped channel, and/or by the insertion of a threaded post downwardly between the upwardly extending members.
It has been recognized that if only one of the locking elements, i.e., the threaded nut on the outside of the channel, or the threaded post betweeen the upwardly extending members, is used, under normal loading of force, the upwardly extending members which form the U-shaped channel therebetween can deflect, thereby loosening the rod in the channel. (If the threaded nut is used, the members may deflect inwardly; if the threaded post is used, the members may deflect outwardly.)
A variety of solutions to this problem have been proposed, generally including a dual locking assembly having both an outer locking nut and an inner threaded post. For example, the device described in U.S. Pat. No. 5,443,467 to Biedermann, et al. teaches a locking mechanism which includes a threaded post which is inserted down the central axis of the U-shaped channel in head portion of the pedicle screw, and a threaded nut which is also applied to the outer surface of the channel portion of the head. The use of the Biedermann, et al. device is tedious inasmuch as the dual locking mechanisms must be applied simultaneously, for if one is positioned first, the second will not engage the threading (because of a forced deflection of the upwardly extending members outwardly by the threaded post, and inwardly by the threaded nut). In addition, the multiplicity of pieces which must be coordinated and accounted for during a given operation, as well as the size of such pieces, are considerable disadvantages of assemblies of the type discribed in the Beidermann, et al. reference.
It is, therefore, the principal object of the present invention to provide a unitary locking mechanism which securely retains the rod in the channel of the head of a pedicle screw.
In addition, it is an object of the present invention to provide such a locking mechanism which has only one element so as to reduce the number of pieces which must be utilized during a given surgical procedure.
Accordingly it is also an object of the present invention to provide a locking means which eliminates a class of small parts which are tedious to manipulate and are a risk of misplacement during a surgical procedure.
Other objects of the present invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter.